So, armed with some good old black electrician duct tape, I covered all the unprotected MB connectors.

On my specific case, I reckon the problem was related to the power connector: because of the liquid spill, the power connector "sponge" absorbed all the liquid and it reduced in size (and so it didn't protect the power connector anymore).

By screwing the bottom cover to the case, somehow, the metal from the back cover touched the power connector (this should also explain the erratic shutdown...).